Accurate temperature measurement is essential to many fields. It is particularly important in the medical and veterinary fields, where internal temperature measurements indicate the health of a patient, or how the patient is reacting to various conditions. Measuring the internal temperature of a part of a patient's body may be accomplished by implanting, or ingesting, a temperature responsive transmitter.
Temperature responsive transmitters generally utilize a temperature sensitive component which varies in some physical characteristic, such as resistance, inductance, or capacitance, as temperature changes. This variation, in turn, is employed to change the frequency of the transmitter. Factors of interest which affect the design of a temperature responsive transmitter include linearity with temperature, frequency stability, frequency range, power consumption, output power, temperature range, accuracy, physical size, and cost.
Typical components which have been utilized in temperature responsive transmitters as temperature sensitive components to cause the frequency of the transmitter to change with temperature are thermistors, diodes, and piezoelectric crystals. Where a thermistor is used to vary the frequency of a transmitter, linearity and stability have proved to be unsatifactory. Where a diode is used, the voltage across the diode varies with temperature, and this voltage change must be converted to a change in frequency. To change the diode voltage to frequency requires complex circuitry, and the linearity of conversion is generally not satisfactory. The complex circuitry also increases the size of the transmitter which makes it more suitable for external use rather than internal use. When a piezoelectric crystal is used as the frequency determining element of a transmitter, linearity and frequency stability are satisfactory if the crystal is specially cut to have a large linear temperature coefficient. Previously, these specially cut crystals were relatively expensive, and this limited their use to situations in which the probability of loss of the temperature responsive transmitter was low.